Protagonist

The protagonist is the main character in a story. In a game, it is the player's character.

Doom
Doom's protagonist is left nameless in all installments of the core Doom franchise. In Doom 3, the main campaign, Resurrection of Evil, and The Lost Mission all feature separate protagonists; whereas it is implied the same marine went through Ultimate Doom, Doom II, and Final Doom.

The Doom and its sequel Doom Eternal both share a protagonist named the "Doom Slayer" by the demons. It is presently left ambiguous whether or not he has any relationship to the original games' protagonist, with titles external to the series such as  suggesting it much more heavily than the other currently existing games.

Several mods give an identity to the player character. A few notable examples:
 * Icarus: Alien Vanguard and Daedalus: Alien Defense: Dave Razor, Space Marine, named by.
 * Mordeth: Norman Ash, who got trapped in Hell after severing the invasion portal from the wrong side.

Heretic

 * Corvus is the name of the player character given in ; who is said to be the vanquisher of D'Sparil.

Hexen

 * Baratus the fighter, Daedolon the mage, and Parias the cleric are the available characters.

Strife

 * Strifeguy is said to be a mercenary.

Other games

 * Chex Quest's protagonist is named Fred Chexter and often referred to as the Chex Warrior.
 * Hacx features Danny Evanger, a hacker and ex-convict.
 * Harmony is named after the player's character.